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There's No Such Thing as the Fat-Burning Zone

Michael BoyleMay 26, 2006

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The "fat-burning zone" is another one of those urban legends of fitness. Legend has it, a certain level of exercise results in a larger number of calories burned being derived from fat. This does not mean that stored body fat is the selective source. It only describes the relative percentage of utilization of three substrates: fat, carbohydrate and protein.

Truth: The fat-burning zone actually describes the percentage of calories burned that are derived from fat as an energy source. Do you know when you are burning the most calories from fat? Sorry, but the highest percentage of fat utilization is at rest. The more intense the exercise becomes, the more carbohydrate is used as a source.

Losing weight is about the number of calories burned, not the number of those calories that come from fat as a source. If the fat-burning zone idea actually worked, we could get extremely lean by simply sitting still.

Let's use a mathematical example: Let's assume that we have two identical exercisers who are going to exercise for 20 minutes. Exerciser #1 is doing a slow walk to stay in the "fat-burning zone." Exerciser #2 is running hard for the whole 20 minutes.

To keep the example simple, we will assume that exerciser #1 will derive 40 percent of his or her calories from fat. Exerciser #2 will move out of the fat-burning zone and only derive 20 percent of his or her calories from fat.

Exerciser #1 will walk at three miles per hour and will cover one mile in twenty minutes. This will result in a caloric expenditure of 100 calories, with 40 calories coming from fat.

Exerciser #2 will run at 7.5 miles per hour and will cover 2.5 miles in twenty minutes. This will result in a caloric expenditure of 250 calories with 50 calories coming from fat.

The exerciser in the "fat-burning zone" burned fewer calories and fewer calories from fat in the same amount of time, while the exerciser working harder with disregard for the fat-burning zone burned two-and-a-half times as many calories and 10 more calories from fat.

Now, if we use the calculation of one pound of body fat equaling 3500 calories, the math gets crazy after one year. Let's assume that these exercisers followed the same program for one year and exercised three times a week, exerciser #1 would burn 300 calories a week or 15,600 calories per year. This would result in a fat loss of about 4.5 pounds. Exerciser #2 would burn 39,000 calories and lose 11 lbs by getting out of the old fat burning zone.

Stop worrying about burning fat, and start worrying about burning calories by training harder, smarter and more often.

Michael Boyle is one of the world’s leading experts in the area of performance enhancement and the owner of StrengthCoach.com. He is also the author of Functional Training for Sports.